DEC-205 is an endocytic receptor expressed by a distinct CD8&#945;+ dendritic cell subpopulation. A member of the mannose receptor family, DEC-205 endocytoses antigens and allows them to be presented on MHC class I and II. Targeting of proteins to DEC-205 through chimeric antibody constructs causes clonal deletion or anergy of antigen-specific CD4+ and CD8+ T cells in immunologically normal mice. Recent work (Mukhopadhaya A, et al. PNAS 2008) has shown that beta cell-specific CD8+ T cells can be depleted from NOD mice (which develop autoimmune diabetes around 12-15 weeks of age). We are now assessing the ability of DEC-205 targeting to deplete or anergize CD4+ T cells in autoimmune NOD mice.[unreadable] NOD mice were injected with BDC2.5 T cell receptor (TCR) transgenic CD4+ T cells that recognize an unknown beta cell antigen, followed by treatment with anti-DEC-205 attached to a BDC mimeotope peptide. Three days after anti-DEC-205 treatment, BDC T cells had proliferated in the spleen, peripheral lymph nodes and pancreatic lymph nodes, similar to results in normal mice. However, ten days after treatment with anti-DEC-205, when T cells are deleted in normal mice, antigen-specific cells remained in the autoimmune mice and were not anergic (they retained capacity to produce interferon-&#947;). Given that tolerance can be broken by injection of toll-like receptor (TLR) ligands in normal mice, we hypothesized that endogenous TLR ligands could be responsible for changing the phenotype of dendritic cells in the autoimmune mice. We therefore tested if MyD88-deficient mice would delete the T cells. Ten days after treatment with anti-DEC-205, similar levels of antigen-specific T cells remained in NOD and NOD.MyD88-/- mice, although increased proliferation from CpG treatment of mice was only observed in NOD mice. Therefore, endogenous Myd88-dependent TLR ligands, are not inhibiting DEC-mediated induction of CD4 tolerance in NOD mice. We are currently examining a second beta cell-specific TCR transgenic line that recognizes insulin, as well as looking at cell signal strength through utilization of a lower-affinity BDC mimeotope attached to anti-DEC-205.[unreadable] Another type of DC, Plasmacytoid DCs (pDC) have traditionally been identified as a distinct subset of DCs involved as an innate immune cell in viral immunity due to their expression of TLR7/TLR9 and their ability to secrete large amount of type 1 IFN upon stimulation. However, more recently, pDCs have also been shown to present antigen to CD4 T cells and to be vital in the induction of immune tolerance. We are now addressing the role of pDC in the pathogenesis of autoimmune diabetes by 1) studying the number of pDCs at different stages of diabetes progression in the NOD mice and 2) depleting pDCs in NOD.scid mice that have been induced to develop diabetes.